Article II Section 1 of the Constitution:
Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.
Bet you didn't know you didn't have a right under the US Constitution to vote for president, or more specifically for electors, did you? That comes down from your state.